hen the Golden Gate Bridge was once again placed on high alert last month, the official explanation contained the now-familiar references to intelligence chatter but ''no specific threat.'' Days passed, and the mighty span was of course still standing.

But the concern about the San Francisco landmark was typical of the guessing game that has gone on across the nation in the year since Sept. 11: Where might terrorists strike next?

Cities, for their concentration of people and because they are centers of government and commerce, are at the top of the list, professionals in security and risk analysis say. But a strike at a large suburban shopping mall would prompt panic and fear as well, suggesting a sinister infiltration into everyday America. The destruction of the Statue of Liberty would have symbolic weight but probably insufficient casualties for a terror group's aims, security specialists say.

The Sept. 11 selection of the World Trade Center and the Pentagon as targets, and reportedly the White House and the Capitol as well, is only one clue in the dizzying business of predicting attacks, said Stephanie King, director of risk analysis for Weidlinger Associates, a New York-based consulting firm.

''You can model the likelihood of an earthquake or a hurricane based on science,'' said King, who, like most of the cutting-edge specialists analyzing terrorism today, does work for the insurance industry. ''This is more subjective. It's trying to apply probabilistic methods to something that has to do with a person's mind-set, and that's difficult. There are no neat models like with natural hazards.''

That hasn't stopped analysts from at least trying to define the universe of possible targets, however. A Boston-based company, Applied Insurance Research, made headlines this week when it unveiled a national database of likely terror targets. The company, project manager Jack Seaquist said, assembled a team of former FBI and CIA agents and other veterans of foreign affairs, security, and intelligence, then folded their predictions into probability rankings using a military number-crunching process known as the ''Delphi method.''

High-visibility corporations, the tallest buildings in major cities, government facilities, and so-called trophy locations - the White House, the Golden Gate Bridge, Disneyland - were high on the list.

The information - a landscape of fear in an Excel file - will be used by insurance companies as they set premiums and coverage in the post-Sept. 11 world; it may also be used by companies deciding whether to remain in the city or have backup facilities in the suburbs. The parsing of targets is of particular interest to the architects and builders who are attempting to fortify America, by creating stronger structures and protecting their perimeters from explosive-laden vehicles.

Taking such steps in the physical realm is expensive and time-consuming. And while probability rankings - and common sense - provide some guidance, there is no clear rationale for designing some places and structures for defense while leaving others without armor.

''The Cold War offered a clear kind of targeting calculus: They'd hit our big cities, or later, our missile sites; we'd hit their big cities, or their missiles,'' said Tom Vanderbilt, author of ''Survival City,'' an examination of the physical responses to the Soviet nuclear threat.

''Now, if everything is presumed to be a target, how do you assign a protective capability to one thing over another?''

The new era of homeland security is similar to the Cold War in the sense that a terror group's nuclear device would almost certainly be detonated in a city - on a motorboat in Boston Harbor, for example. Similarly, a biological or chemical assault would be most effective in places where there are large concentrations of people and limited means of escape.

Anticipating such scenarios has changed the way Americans think about their relationship to the physical environment, said Alex Krieger, principal in the Cambridge architectural firm Chan Krieger & Associates and chairman of the urban planning and design department at the Harvard Design School.

But unlike the citizenry in the London Blitz during World War II - some hunkered down in shelters, others sent their children to the country - Americans have made no similar calculations. Given the vagueness and uncertainty about targets, Krieger said, people either refuse to worry about the safety of a particular geographical location or think about it in a reactive, of-the-moment fashion.

Recalling a recent visit to Manhattan, Krieger said he was pleased to see ''thousands of people going about their business, and there was a sense of perseverance and normalcy.'' But there was also no avoiding an awareness of being in the city that was the scene of such unfathomable death and destruction a year ago, he said. ''Certainly people are more sensitive, and there is concern about the safety of downtowns,'' he said.

Then again, if the next attack is at a location such as Microsoft's office campus in suburban Redmond, Wash., Krieger said, ''Our unease about going into tall buildings might diminish.''

Concern about security shifts in tandem with the most recent event. In the new age of terrorism, Krieger said, ''Our intuition tells us only that it's always safer someplace else.''